1. Field of the Invention
This invention relates to a method and apparatus for irradiating a patient and, more particularly, to a novel and highly-effective method and apparatus for radiation therapy for cancer patients.
2. Description of the Prior Art
Cancer cells can be weakened and ultimately killed by bombardment with certain kinds of radiation, and radiation therapy is an important treatment for cancer. A problem with radiation therapy is that the radiation that kills the cancer cells also injures and may kill normal cells, so that the patient's body is damaged by the treatment.
To minimize this problem, it is conventional to irradiate a patient at different angles. For example, a patient may be irradiated for three or four minutes per session in a number of sessions spread over a period of weeks, the radiation being directed in each session along a different radiation axis. The tumor is made the "pivot" or "fulcrum" of the radiation beam as it were, so that each radiation dose passes through the tumor, the cells of which will be heavily damaged and possibly killed. Normal cells, on the other hand, receive much smaller radiation doses, from which they are expected to recover.
Because of the damage that radiation is capable of doing to normal cells, it is desirable to ascertain the position of the tumor within the body to an accuracy of a millimeter or less. It is therefore desirable that the patient assume a standard position so that the effects of gravity on the position of internal organs is minimized. The standard position for radiation therapy is typically the supine position, although the prone position and other recumbent positions can also be employed. The patient must remain quite still during the radiation therapy for a period that may for example be three or four minutes.
The radiation may consist of charged particles, neutrons or photons. Neutrons have been found particularly efficacious for the treatment of certain types of prostatic cancer. In any case, one type of machine constituting a radiation source comprises a floor-mounted acceleration chamber for accelerating charged particles and a gantry connected thereto and having the shape of a gooseneck. The gantry can be swivelled about a horizontal axis so that the radiation emitted through the "mouth" of the gooseneck is directed substantially radially inward towards the patient from any selected position in an arc concentric with a point in the patient. This makes it possible to irradiate a tumor from various angles, thereby possibly killing the cancer cells while sparing the surrounding normal cells.
Of course, the gantry is massive, because it must contain large coils and heavy magnets for bending the path of the charged particles as they travel through the gooseneck. Consequently, the gantry normally requires an equally massive counterweight to facilitate its swivelling movement. If neutrons are to be generated, the gantry must also contain a target, which in this machine design must be located substantially in the mouth of the gantry, since the direction of travel of a neutron can not be changed by a magnetic or electrostatic field and is determined by the dynamics of the collision between the charged particles and the target.
In another type of machine the acceleration chamber itself is mounted in a gantry, which can be gooseneck-shaped or a drum, etc., and moves around the patient in order to irradiate the patient from various angles. This also requires a counterweight, since the machine is massive.
The machine may be a cyclotron, synchro-cyclotron, synchrotron, or microtron. Also, the coils of some such machines for generating the magnetic field may be superconducting or not. If superconducting, the size and weight of the machine is significantly reduced.
Even in the most favorable case, however, the total mass of the machine is measured in tens of tons and the cost in millions of dollars (1985 prices). In addition, such a bulky and massive installation requires a substantial amount of floor space and a substantial ceiling height and may require strengthening of building supports, etc., so that the total cost of the installation substantially exceeds the purchase price of the machine itself.
Examples of the prior art include U.S. Pat. Nos. 3,925,676, to Bigham to Nunan 4,112,306, and to Rautenbach 4,139,777. The Bigham and Nunan patents disclose cyclotrons, the coils of which are respectively superconducting and nonsuperconducting, as a source of neutrons for medical therapy. The cyclotrons are moved orbitally about a stationary patient, whereby the patient can be irradiated from different angles.
The Rautenbach patent discloses a linear rail whereby a cyclotron may be rolled so that it projects a beam in different directions; means whereby a beam of charged particles can be extracted at different circumferential points of an accelerator chamber; and a pivot beam whereby a cyclotron can be moved in an orbital path about a horizontal pivot axis in a manner such that the radiation axis of a neutron beam intersects the horizontal pivot axis, where the patient is positioned.
All of these structures are massive and ideally require equally massive counterweights to facilitate the described orbital movements of the radiation sources. This results in a substantial cost not only for the installation itself but also for space rental, building strengthening, etc., as indicated above.